Steel Slag and Autoclaved Aerated Concrete Grains as Low-Cost Adsorbents to Remove Cd²⁺ and Pb²⁺ in Wastewater: Effects of Mixing Proportions of Grains and Liquid-to-Solid Ratio

Abstract

This study investigated the applicability of industrial by-products such as steel slag (SS) and autoclaved aerated concrete (AAC) grains (<0.105, 0.105–2, 2–4.75 mm) as low-cost adsorbents for simultaneous removal of Cd²⁺ and Pb²⁺ in wastewater. A series of batch adsorption experiments was carried out in single and binary-metal solutions of Cd²⁺ and Pb²⁺ by changing the mixing proportions of SS and AAC grains. In addition, the effect of the liquid-to-solid ratio (L/S) on the removal of Cd²⁺ and Pb²⁺ in multi-metal solution was examined. Results showed that SS grains had a high affinity with Cd²⁺ in the single solution, while AAC grains had an affinity with Pb²⁺. In the binary solution, the mixtures of SS and AAC grains removed both Cd²⁺ and Pb²⁺ well; especially, the tested adsorbents of SS+AAC [1:1] and SS+AAC [1:4] mixtures achieved approximately 100% removal of both metals. Based on the results in the multi-metal solutions, the metal removal % and selectivity sequence varied depending on the mixed proportions of SS and AAC grains and L/S values. It was found that the SS+AAC [1:1] mixture of SS and AAC grains showed 100% removals of Cd²⁺, Pb²⁺, Cu²⁺, Ni²⁺, and Zn²⁺ simultaneously at L/S = 10 and 60.

1. Introduction

Due to rapid urbanization and industrialization, the discharge of wastewater containing toxic heavy metals from various sources such as industries, mines, vehicles, batteries, and metal-containing paints is increasing all over the world, especially in developing countries [1,2]. Heavy metal ions are non-biodegradable and tend to accumulate in living organisms, and those are considered toxic or carcinogenic ions [3,4]. Over the past five decades, for example, the annual global release of heavy metals reached 22,000 tons for cadmium, 939,000 tons for copper, 783,000 tons for lead, and 1,350,000 tons for zinc [5]. Inadequate treatments of toxic heavy metals such as Cd²⁺ and Pb²⁺ cause not only serious surface and groundwater pollutions and soil contamination [6,7,8] but also various human diseases including acute or chronic poisoning, dermatitis, brain damage in children, and digestive tract cancer [9,10,11]. Furthermore, the direct discharge of Cd²⁺ and Pb²⁺ into the sewage system causes a negative impact on the effectiveness of biological wastewater treatment [12]. Thus, Cd²⁺ and Pb²⁺ can be considered the most commonly available and most harmful heavy metals to humans as well as the environment. Thus, proper treatment of these heavy metals in wastewater is essential to protect public health and the environment.

Numerous efforts are being made to develop cost-effective innovative methods of wastewater treatment, such as hydroxide precipitation [13], membrane filtration [14], reverse osmosis [15], phytoextraction [16], ion exchange [17], electrokinetic remediation [18], and adsorption [19]. When developing new methods, economic feasibility and sustainability are given high priority. However, adsorption has received a great deal of attention because of its high efficiency and cost effectiveness [20]. At present, a number of adsorbents are used for heavy metal removal on experimental as well as industrial levels. Out of those, activated carbon is the most widely used adsorbent due to its high adsorption capacity [21]. However, activated carbon is still in limited use because of economic reasons, mostly in developing countries [22]. Therefore, the evaluation of the effective use of low-cost and locally available adsorbents is receiving much attention. Low-cost adsorbents are classified generally as geo-sorbents [23,24,25,26,27], bio-sorbents [28,29,30,31,32], industrial wastes [33,34,35], industrial by-products (IBPs) and construction and demolition waste (CDW) [36,37,38,39,40,41,42], and modified low-cost adsorbents [19,21,43,44,45,46]. Many adsorption experiments have been performed to examine their adsorption capacities and characteristics [47]. However, among these low-cost adsorbents, IBPs and CDW are still marginalized in adsorption studies as well as industrial applications, although those materials have the same potential as other low-cost adsorbents to adsorb heavy metals from wastewater [36,37,38,39,48,49,50]. Therefore, more studies on the effective use of IBPs and CDW are required.

The growing industrialization in developing countries, on the other hand, generates large amounts of IBPs such as steel slag (SS) by the steel-making industry, coal ash by coal-fired power plants, and mining waste by extracting and processing mineral resources [51,52]. Along with the rapid generation of IBPs, improper treatment and uncontrolled dumping of IBPs are becoming a major environmental issue in many developing countries [53,54]. Moreover, with increasing production of non-fired bricks such as cement blocks and autoclaved aerated concrete (AAC) all over the world, the amount of scrap waste has also been increasing in developing countries [55,56]. The AAC scrap waste is not fully reused in most of the countries but dumped on-site without any treatment. Proper management is vital for sustainable development, and effective utilizations of IBPs and scrap waste of non-fired bricks are required for circulating those materials in society. Hence, it is timely and important to promote the reuse of these abundantly available waste materials as a cost-effective adsorbent for wastewater treatment process to give added value to IBPs and AAC scrap waste.

Until now, many studies have investigated the applicability of IBPs and other types of low-cost adsorbents for heavy metal treatment processes, but most of the studies mainly target the treatment of heavy metals at low concentrations (typically, heavy metal concentrations <200 mg/L) [21,57,58,59]. Furthermore, most of these studies examined the adsorption capacities of target adsorbents in single and/or binary-metal solutions at a constant liquid-to-solid (L/S) ratio [50]. Because actual industrial wastewater might be contaminated by multi-species heavy metals like Pb²⁺, Cd²⁺, Ni²⁺, Cu²⁺, and Zn²⁺, further studies are needed to examine the simultaneous removal of heavy metals fully considering the co-existence of competitive metals in wastewater [19,60]. For example, it has been reported that Cd²⁺ adsorption onto cementitious adsorbents, including AAC grains, was hampered by the co-existence of Pb²⁺ and Cu²⁺ due to their high affinities with the tested adsorbents [61,62,63,64]. In addition, Kumara et al. 2019c [65] examined the simultaneous removal potential of Cd²⁺ and Pb²⁺ in the multi-metal solutions by AAC, SS, and AAC+SS [1:1] grains and revealed selectivity sequences of Pb²⁺ > Cu²⁺ > Ni²⁺ > Zn²⁺ > Cd²⁺ for AAC, Pb²⁺ > Cu²⁺ > Cd²⁺ > Ni²⁺ > Zn²⁺ for SS, and Pb²⁺ ≈ Cu²⁺ > Cd²⁺ > Zn²⁺ > Ni²⁺ for AAC+SS [1:1] under an L/S ratio 60. Thus, many researchers have concluded that the simultaneous removal of Cd²⁺-like metals using a low-cost adsorbent is impossible. Therefore, it is worthwhile to discover low-cost adsorbents that have potential for simultaneous removal of commonly available heavy metals in wastewater. In order to examine the simultaneous removal of Cd²⁺ and Pb²⁺ in wastewater, therefore, this study investigated the adsorption characteristics of those metals onto the mixtures of SS and AAC grains in single and binary-metal solutions. Especially, the selectivity sequence on metal adsorption was carefully investigated in the multi-metal solutions (coexistence of Ni²⁺, Cu²⁺, and Zn²⁺) at different L/S ratios ranging from 5 to 250.

2. Materials and Methods

2.1. Adsorbents Preparation and Characterization

Commercially available steel slag (SS) for civil engineering applications (Nippon Steel Cooperation and Sumitomo Metal Industries, Ltd., Saitama, Japan) and autoclaved aerated concrete (AAC) (Asahi Kasei Construction Material Corp., Tokyo, Japan) were used. General information on the manufacturing and characteristics of SS and AAC are given in Nippon Slag Association [66] and Trong et al. [67]. After crushing by hand in the laboratory, the tested samples were sieved to three grain fractions of <0.105, 0.105–2, and 2–4.75 mm. The dataset of material properties and the adsorption tests for AAC grains analyzed in this study were obtained from Kumara et al., 2019a [61].

The basic physical and chemical properties of SS and AAC grains are summarized in

Table 1. Basic physical and chemical properties of tested adsorbents.

Adsorbent	Particle Size (mm)	pH		EC (mS/cm)	LOI (%)	wAD (%)	Gs	SSA (m2/g)	Reference
		DI	1M KCl
	<0.105	12.9	13.1	6.1	14.5	2.5	3.05	8.0
SS	0.105–2	12.4	12.6	6.9	4.8	1.9		4.9	This study
	2–4.75	13.1	13.1	5.0	4.8	1.9		2.6
	<0.105	10.0	8.7	1.7	10.3	4.4		23.6
AAC	0.105–2	10.0	8.3	1.8	10.8	3.9	2.49	23.6	[61]
	2–4.75	9.9	8.9	1.4	9.8	4.0		21.9

. The Brunauer–Emmett–Teller (BET) surface area (SSA) was measured by a ASAP2020 adsorption analyzer (Micromeritics, Norcross, GA, USA). Measured specific gravity (G_s) values of SS grains were higher compared to AAC grains, and pH values showed that SS grains are alkaline in water due to the hydration reaction of CaO with a release of OH-. The SSA values of SS grains were lower than those of AAC grains and decreased with decreasing grain size. This implied that SS grains had fewer inter-grain pores compared to AAC grains, and the outer-grain surface controlled the SSA of SS grains.

The chemical composition of tested adsorbents was characterized by energy-dispersive X-ray spectroscopy (EDX; X-Max Extreme, Oxford Instruments, High Wycombe, UK) and X-ray diffractometry (XRD; XRD-7000, Shimadzu Cooperation, Kyoto, Japan). The EDX test data are shown in

Table 2. Chemical composition of tested adsorbents.

Adsorbent	Composition (wt %)							Reference
	SiO2	CaO	Al2O3	Fe2O3	K2O	MgO	Others
SS	16.8	41.7	3.2	22.5	0.2	3.5	12.1	This study
AAC	48.6	33.8	2.8	1.9	0.5	0.4	12.0	[61]

. Higher CaO (41.7%) and Fe₂O₃ (22.5%), and lower SiO₂ (16.8%) were found for SS grains compared to those of AAC grains, indicating a potential for the ion exchange (Ca²⁺) reaction with heavy metals in wastewater like other calcium silicate materials [68,69]. The XRD analysis showed that magnetite (Fe₃O₄), iron oxidate (FeO), calcite (CaCO₃), calcium hydroxide (Ca(OH)₂), and halloysite (Al₂Si₂O₅(OH)₄) were the main minerals in SS grains. Both EDX and XRD analyses confirmed the existence of high amounts of metal oxides and hydroxides in SS grains, implying that SS grains favored the adsorption of heavy metals such as Cd²⁺, Cu²⁺, and Pb²⁺ [70,71,72].

2.2. Procedures of Batch Experiments

Stock solutions of Cd²⁺, Pb²⁺, Cu²⁺, Ni²⁺, and Zn²⁺ (synthesized wastewater) were prepared by dissolving CdCl₂, PbCl₂, CuCl₂, NiCl₂, and ZnCl₂, respectively, in deionized water (DI). The solutions’ pH and ionic strength were controlled using 1N HCl, 1N NaOH, and NaNO₃. All chemicals used for this study were analytical grade from Wako Pure Chemical Industries, Ltd., Osaka, Japan, with more than 98% chemical purity. A standard batch method recommended by the Organization of Economic Cooperation and Development [73] was used for all batch adsorption experiments. The concentrations of heavy metals and other ions were measured using a flame atomic absorption spectrophotometer (AAS; AA 6200, Shimadzu, Japan), the exact pH and EC values for each metal solution were measured before the experiment, and their changes were observed during the experiments using a pH/EC meter.

Test conditions of adsorption experiments are summarized in

Table 3. Summary of batch adsorption experiments and testing conditions.

Adsorbent	Types of Experiment	Liquid-to-Solid Ratio (L/S)	Initial Metal Concentration, (Ci = mg/L)
SS	1. Adsorption isotherm in single-metal solution (Cd2+, Pb2+)	60 (Cd2+), 10 (Pb2+)	0–5000 (Cd2+), 0–1500 (Pb2+)
SS SS+AAC [4:1] SS+AAC [1:1] SS+AAC [1:4] AAC	2. Removal % in binary-metal solution (Cd2+ + Pb2+)	60	1000
	3. Removal % in multi-metal solution (Cd2+ + Pb2+ + Cu2+ + Ni2+ + Zn2+)	60	1000
	4. Effect of L/S ratio on removal % in multi-metal solution	5, 10, 60, 100, 250	1000

Note: Deionized water (DI) with natural pH was used as a background solution for all experiments.

. The types of batch adsorption experiments in this study were categorized into four groups to investigate: (1) adsorption isotherm in a single-metal solution with L/S = 60 for Cd²⁺ and 10 for Pb²⁺, (2) % removal in a binary-metal solution (Cd²⁺, Pb²⁺) with L/S = 60, (3) % removal in multi-metal solution (Cd²⁺, Pb²⁺, Cu²⁺, Ni²⁺, Zn²⁺) with L/S = 60, and (4) the effect of L/S ratios on % removal in a multi-metals solution with different L/S ratios = 5–250.

Three grain sizes of adsorbents of <0.105, 0.105–2, and 2–4.75 mm were used for the adsorption isotherm experiments in a single-metal solution, and an adsorbent grain size of 0.105–2 mm was used for other adsorption experiments. All test conditions were carried out with triplicate measurements, and the averaged values were given in the paper because of a small variation of measured data.

2.2.1. Adsorption Isotherms for Cd²⁺ and Pb²⁺ in a Single-Metal Solution

Adsorption isotherm experiments for Cd²⁺ and Pb²⁺ onto SS and AAC grains were carried out at natural pH with the initial metal concentration (C_i) of 0–5000 mg/L for Cd²⁺ and 0–1500 mg/L for Pb²⁺ (19 different concentrations, fully considering the actual metal concentration range in industrial wastewater [74]) at L/S = 60. To determine the maximum adsorption capacity and intensity of Cd²⁺ and Pb²⁺ onto the tested adsorbents, Langmuir (Equation (1)) and Freundlich (Equation (2)) models were used to fit the obtained experimental data:

C_e/Q_e = 1/(bQ_m)+ C_e/Q_m

(1)

log Q_e = log K_f + 1/n (log C_e)

(2)

where C_e (mg/L) is the equilibrium concentration of heavy metals, Q_e (mg/g) is the amount adsorbed per adsorbent at the equilibrium, b (g/L) is the Langmuir constant related to binding strength, Q_m (mg/g) is the maximum adsorption capacity, K_f (L/g) is the Freundlich adsorption capacity, and 1/n is the adsorption intensity.

2.2.2. Effect of Competitive Metal Ions on Cd²⁺ and Pb²⁺ Adsorption in Binary Metal Solution

To examine the effect of competitive metal ions on Cd²⁺ and Pb²⁺ adsorption, batch experiments were carried out in a binary-metal solution of Cd²⁺ and Pb²⁺ at natural pH with C_i = 1000 mg/L. Five different mixtures of SS and AAC grains, i.e., mixing proportions of SS (alone), SS+AAC [4:1], SS+AAC [1:1], SS+AAC [1:4], and AAC (alone), were used as tested adsorbents. The ratios in brackets show the mixing proportions in weight % (e.g., (1:1) means a mixture of 50% SS and 50% AAC). In binary-metal adsorption experiments, the mixed solutions of Cd²⁺ to Pb²⁺ were used at metal molar ratios of 1:0, 1:0.25, 1:0.5, 1:0.75, 1:1, 1:2, and 1:5 to investigate the effect of Pb²⁺ concentrations on Cd²⁺ adsorption. Vice versa, the same mixing solutions of Pb²⁺ and Cd²⁺ solutions were used to investigate the effect of Cd²⁺ on Pb²⁺ adsorption. In each experiment, the metal removal percentage (R, %) was calculated using Equation (3) [46]:

R = [(C_i–C_e)/C_i] × 100

(3)

2.2.3. Effect of Competitive Metal Ions on Cd²⁺ and Pb²⁺ Adsorption in Multi-Metal Solutions

To examine the effects of competitive metals on Cd²⁺ and Pb²⁺ adsorption onto tested adsorbents with different L/S, batch adsorption experiments in multi-metal solutions were carried out using a mixed metal solution of Cd²⁺, Pb²⁺, Cu²⁺, Ni²⁺, and Zn²⁺ at natural pH with C_i = 1000 mg/L. Like the adsorption experiments of binary-metal solutions, five different mixtures of SS and AAC grains (SS, SS+AAC [4:1], SS+AAC [1:1], SS+AAC [1:4], and AAC) were used with five different L/S values of 5, 10, 60, 100, and 250. In each experiment, the R value of each metal was calculated using Equation (3).

3. Results and Discussion

3.1. Adsorption Isotherms for Cd²⁺ and Pb²⁺ in Single Metal Solution

The measured adsorption isotherms for Cd²⁺ and Pb²⁺ onto SS grains with different grain sizes are shown in Figure 1. In the figures, the Langmuir model (Equation (1)) was well-fitted to the measured data except for the Cd²⁺ adsorption onto SS grains with <0.105 mm (Figure 1a). The SS grain with <0.105 mm showed a very high Cd²⁺ adsorption and did not show the maximum adsorption capacity (Q_m) in the range of C_i = 0–5000 mg/L. For both Cd²⁺ and Pb²⁺ adsorptions, the adsorption decreased with increasing grain size. This can be understood to indicate that the adsorption surface area of tested adsorbents controlled the adsorption capacity, and the sample of <0.105 mm with higher SSA imposed a higher adsorption capacity compared to the samples with lower SSA of the grain sizes of 0.105–2, 2–4.75 mm (Table 1). The Freundlich model (Equation (2)), on the other hand, fitted well with all tested adsorbents in the range of C_i = 0–5000 mg/L for Cd²⁺ and 0–1500 mg/L for Pb²⁺. This suggests that a monolayer adsorption is predominant at the early stage of the adsorption process (typically, C_i < 1500 mg/L) and the adsorption process shifts to a multilayer adsorption, especially at higher than C_i > 1500 mg/L, for both Cd²⁺ and Pb²⁺ [75,76].

The fitted adsorption parameters by Freundlich and Langmuir models for tested SS grains are summarized in

Table 4. Fitted Langmuir and Freundlich model parameters for Cd²⁺ and Pb²⁺ adsorption onto tested adsorbents.

Adsorbent	Metal	Particle Size (mm)	Langmuir			Freundlich			Reference
			Qe (mg/g)	b (L/mg)	r2	Kf (mg/g)	1/n	r2
SS	Cd2+	<0.105	3131	–	–	105	0.27	0.98	This study
		0.105–2	244	0.18	0.99	114	0.17	0.89
		2–4.75	130	0.09	0.99	92.0	0.04	0.77
	Pb2+	<0.105	17.5	0.02	0.97	0.57	0.67	0.97
		0.105–2	8.6	0.01	0.91	0.15	0.70	0.96
		2–4.75	8.2	0.01	0.93	0.13	0.72	0.94
AAC		<0.105	16.5	1.72	0.99	9.89	0.11	0.91	[61]
	Cd2+	0.105–2	16.5	0.91	0.99	9.92	0.10	0.93
		2–4.75	15.2	0.69	0.99	8.82	0.11	0.89
		<0.105	2581	–	–	137	0.20	0.97
	Pb2+	0.105–2	2571	–	–	121	0.24	0.98
		2–4.75	250	0.05	0.99	74.3	0.22	0.98

Note: ¹:Q_m values were estimated by the measured maximum adsorption amount. Langmuir and Freundlich parameters for SS were determined fitting the adsorption isotherm of Cd²⁺ at 0 ≤ C_i ≤ 5000 mg/L and the adsorption isotherm of Pb²⁺ at 0 ≤ C_i ≤ 1500 mg/L. Langmuir and Freundlich parameters for AAC were determined fitting the adsorption isotherms of Cd²⁺ and Pb²⁺ at 0 ≤ C_i ≤ 2000 mg/L.

with the reported data for AAC grains [61]. It shows clearly that the SS grains have higher affinities (higher Q_m and K_f) to Cd²⁺ compared to AAC grains while the AAC gains have higher affinities to Pb²⁺ compared to SS grains. These results imply that the mixing of SS and AAC grains would be effective for the simultaneous adsorption of Cd²⁺ and Pb²⁺, as discussed in the following sections. For reference, the measured Q_m values in this study were compared to the previously reported values for different types of adsorbents such as IBPs (including construction and demolition waste) and geo- and bio-sorbents and are summarized in Table 5. It is noticeably shown that SS has the highest Q_m of Cd²⁺ adsorption compared to those of other adsorbents.

Figure 2 shows the relationship between the amounts of Ca²⁺ and adsorbed metal released onto SS grains sized 0.105–2 mm in the measured C_i range. For both Cd²⁺ and Pb²⁺ adsorptions, the Ca²⁺ was released linearly along with the metal adsorption (r² ≥ 0.88). The correlation regression showed that the ratios of released Ca²⁺ became greater than 1 (2.24 for Cd²⁺ and 6.37 for Pb²⁺). This means that approximately 2 and 6 times of Ca²⁺ were released when the one metal was adsorbed, indicating the adsorption mechanism was not controlled by a simple 1:1 ion exchange process of Ca²⁺ and Cd²⁺/Pb²⁺ on the adsorbent surface of SS grains in the single-metal solution system. On the other hand, Kumara et al. 2019a [61] observed an almost 1:1 relationship between the released Ca²⁺ amount and adsorbed Cd²⁺/Pb²⁺ for AAC grains, and the hydrated adsorbent surface was the dominant metal adsorption mechanism [77,78].

Table 5. Comparison of the maximum adsorption capacity (Q_m) of tested adsorbents with reported values.

Category	Adsorbent	Particle Size (mm)	BET Surface Area (m2/g)	Liquid-to-Solid Ratio (L/S)	pH Range	Cd2+		Pb2+		Ref.
						Ci Range (mg/L)	Qm (mg/g)	Ci Range (mg/L)	Qm (mg/g)
Industrial by-products (IBPs) (including construction and demolition waste)	Steel slag (SS)	0.105–2	4.9	60	10–12	0–5000	130–313	0–1500	8.2–17.5	This study
	Fly ash	0.01–0.02	2.8	50–10,000	2–10	0–5	3.8	0–5	5.1	[35]
	Blast furnace slag	0.15–0.3	1.1	50–10,000	2–10	0–5	5.1	0–5	4.9
	Arc furnace slag	0.9–2	3.42	100	2–8	22.5–360	6.5	41–662	25	[33]
	Autoclaved aerated concrete	0.105–2	23.6	60	8–10	25–2000	16.5	25–2000	257	[61]
	Crushed concrete fines	0.105–2	11.2	60	8–11	25–2000	22.8	25–2000	129
	Crushed clay bricks	0.105–2	15.9	10	6–7	0–2000	3.2	0–2000	5.5	[79]
	Municipal solid waste slag	0.105–2	-	10	8–9	0–2000	2.3	0–2000	3.3
	Cellular concrete	1.5–2.5	177	1428	6.5–7.2	10–200	28.7	10–200	157	[77]
Geo-sorbents	Zeolite	-	15.4	40–1000	6.5–6.7	0–50	6.72	0–50	9.97	[43]
	Ca-bentonite	<0.063	87	1000	≈5	1–100	31.3	5–150	85.5	[80]
	Sepiolite	<0.1	-	100	-	50–600	19.2	50–600	50	[81]
	Limestone	Powder	119	6667	2–6	1–200	52.9	1–200	184	[82]
	Montmorillonite	-	-	100	≈2	10–500	1.2	10–500	3.25	[83]
Bio-sorbents	Coconut coir husk	<0.35	1.83	400	≈7	2.5–200	47.3	25–200	66.1	[30]
	Coconut husk/shell	0.075	212	10	≈8.8	100–2000	3.5	100–2000	13.4	[31]
	Biochar	0.15	51.2	500	5–7	50–900	75	50–900	129	[84]
	Green algae	1–1.5	-	125	2–6	22–382	39.2	41–704	74.3	[85]
	Lemon berry	0.15	-	100	4.8–5	110–2800	129	200–5200	310	[86]

Table 5. Comparison of the maximum adsorption capacity (Q_m) of tested adsorbents with reported values.

Category	Adsorbent	Particle Size (mm)	BET Surface Area (m2/g)	Liquid-to-Solid Ratio (L/S)	pH Range	Cd2+		Pb2+		Ref.
						Ci Range (mg/L)	Qm (mg/g)	Ci Range (mg/L)	Qm (mg/g)
Industrial by-products (IBPs) (including construction and demolition waste)	Steel slag (SS)	0.105–2	4.9	60	10–12	0–5000	130–313	0–1500	8.2–17.5	This study
	Fly ash	0.01–0.02	2.8	50–10,000	2–10	0–5	3.8	0–5	5.1	[35]
	Blast furnace slag	0.15–0.3	1.1	50–10,000	2–10	0–5	5.1	0–5	4.9
	Arc furnace slag	0.9–2	3.42	100	2–8	22.5–360	6.5	41–662	25	[33]
	Autoclaved aerated concrete	0.105–2	23.6	60	8–10	25–2000	16.5	25–2000	257	[61]
	Crushed concrete fines	0.105–2	11.2	60	8–11	25–2000	22.8	25–2000	129
	Crushed clay bricks	0.105–2	15.9	10	6–7	0–2000	3.2	0–2000	5.5	[79]
	Municipal solid waste slag	0.105–2	-	10	8–9	0–2000	2.3	0–2000	3.3
	Cellular concrete	1.5–2.5	177	1428	6.5–7.2	10–200	28.7	10–200	157	[77]
Geo-sorbents	Zeolite	-	15.4	40–1000	6.5–6.7	0–50	6.72	0–50	9.97	[43]
	Ca-bentonite	<0.063	87	1000	≈5	1–100	31.3	5–150	85.5	[80]
	Sepiolite	<0.1	-	100	-	50–600	19.2	50–600	50	[81]
	Limestone	Powder	119	6667	2–6	1–200	52.9	1–200	184	[82]
	Montmorillonite	-	-	100	≈2	10–500	1.2	10–500	3.25	[83]
Bio-sorbents	Coconut coir husk	<0.35	1.83	400	≈7	2.5–200	47.3	25–200	66.1	[30]
	Coconut husk/shell	0.075	212	10	≈8.8	100–2000	3.5	100–2000	13.4	[31]
	Biochar	0.15	51.2	500	5–7	50–900	75	50–900	129	[84]
	Green algae	1–1.5	-	125	2–6	22–382	39.2	41–704	74.3	[85]
	Lemon berry	0.15	-	100	4.8–5	110–2800	129	200–5200	310	[86]

3.2. Removal of Cd²⁺ and Pb²⁺ in Binary Metal Solution

Measured values of the percentage of metal removed, R % (Equation (3)) for Cd²⁺ and Pb²⁺ in the binary-metal solution by tested adsorbents, were plotted against the Cd:Pb and Pb:Cd molar mixed ratios in Figure 3. Like the test results in a single-metal solution for the removal of Cd²⁺, SS had a strong affinity with Cd²⁺ and became independent of the existence of Pb²⁺ in binary-metal solution, SS, and its mixtures (SS, SS+AAC [4:1], SS+AAC [1:1], SS+AAC [1:4]) gave approximately R = 100% (Figure 3a–d,f–i). In contrast, the removal of Pb²⁺ in the binary-metal solution was controlled by AAC, and AAC and its mixtures (AAC, SS+AAC [1:4], and SS+AAC [1:1]), which gave approximately R = 100% (Figure 3c–e,h–j). Previous studies reported that geo- and bio-adsorbents had difficulty achieving the sufficient removal of Cd²⁺ in the presence of Pb²⁺ in the binary-metal solution [30,31]. In this study, however, the tested results suggested strongly that the mixing of SS and AAC grains was effective to simultaneously remove Cd²⁺ and Pb²⁺ in a binary-metal solution; the mixtures of SS+AAC [1:1] and SS+AAC [1:4] were especially able to absorb Cd²⁺(or Pb²⁺) completely, even in the presence of Pb²⁺ (or Cd²⁺) in the solution.

In the batch adsorption tests in the binary-metal solution, the measured pH values after the adsorption (pH_e) ranged from 8–10 for AAC grains (Figure 3e,j) and 10–12 for SS and mixed adsorbents of SS and AAC (Figure 3a–d,f–i). This indicated that the metals were adsorbed onto the tested adsorbents under alkaline conditions. Previous studies suggested that the metal adsorption onto cementitious and adsorbents rich in calcium metal oxides/hydroxides resulted in combined chemical processes and reactions under alkaline conditions: i.e., hydration of the adsorbent surface, hydrolysis of metal ions, physisorption, chemisorption, ion exchange, and surface complexation and precipitation [33,61,77,87,88,89,90]. For example, According to the metal speciation in pH-Eh diagram [91], Pb is presented as Pb²⁺ or possible to precipitate as Pb(OH)₂ in the range of pH = 7–12 and most probably exists as Pb(OH)₃^- in the range of pH >12. On the other hand, Cd exists mainly in the forms of Cd²⁺ and Cd(OH)⁺ in the range of pH = 9–13. As shown in Table 2, the SS and AAC grains tested in this study were rich in CaO and Fe₂O₃. Based on the results of previous studies and the chemical compositions of tested adsorbents, the simultaneous removal of Cd²⁺ and Pb²⁺ from the mixtures of SS and AAC grains in the binary-metal solution under alkaline conditions can be understood as shown in Figure 4. For SS grains, under the given experimental conditions, active hydroxides (-OH) on the surface of metal oxides/hydroxides (especially Fe₂O₃) create a negative surface charge, resulting in the formation of an inner-sphere complex (surface complexation) by replacing Ca²⁺ (high affinity with Cd²⁺). For AAC grains, on the other hand, the ion exchange between Ca²⁺ and Pb²⁺ on the adsorbent surface becomes dominant, and the inner-sphere complex promotes the Pb²⁺ adsorption onto the surface with a negative charge (high affinity with Pb²⁺). Because these two reactions promote the release of Ca²⁺ ions, the amount of Ca²⁺ released became higher than the amount of metal adsorbed, as shown in Figure 2. In addition, hydrated CaO releases high amounts of OH- ions for both SS and AAC grains and promotes the precipitation of metal hydroxides (Cd(OH)₂ and Pb(OH)₂) under the alkaline condition.

3.3. Removal of Cd²⁺ and Pb²⁺ in Multi-Metal Solutions and Selectivity Sequence

Simultaneous removals of Cd²⁺ and Pb²⁺ in multi-metal solutions by the tested adsorbents were examined under different L/S ratios of 5, 10, 60, 100, and 250. The measured R values of metal ions in multi-metal solutions for SS, SS+AAC [1:1], and AAC are exemplified in Figure 5, and selectivity sequences are shown in

Table 6. Selectivity sequence of metal removal, R (%), for tested adsorbents with different liquid-to-solid ratio (L/S).

Adsorbent	L/S	Selectivity Sequence	R (%)
			Cd2+	Pb2+	Cu2+	Ni2+	Zn2+
SS	5	Cd2+ ≈ Cu2+ ≈ Ni2+ > Zn2+ > Pb2+	100	86.1	100	100	99.8
	10	Cd2+ ≈ Cu2+ ≈ Ni2+ > Zn2+ > Pb2+	100	86.9	100	100	99.7
	60	Pb2+ > Cu2+ > Cd2+ > Ni2+ ≈ Zn2+	55.2	99.5	63.4	20.9	20.6
	100	Pb2+ > Cu2+ > Cd2+ > Zn2+ > Ni2+	51.7	76.2	63.8	0.9	8.0
	250	Pb2+ > Cd2+ > Cu2+ > Zn2+ > Ni2+	45.9	62.0	31.7	0.0	6.1
SS+AAC [4:1]	5	Cd2+ ≈ Pb2+ ≈ Cu2+ ≈ Ni2+ ≈ Zn2+	100	100	100	100	100
	10	Cd2+ ≈ Cu2+ ≈ Ni2+ > Pb2+ > Zn2+	100	96.4	100	100	44.5
	60	Cu2+ > Pb2+ > Cd2+ > Zn2+ > Ni2+	46.1	87.8	99.3	17.9	23.2
	100	Pb2+ > Cu2+ > Cd2+ > Zn2+ > Ni2+	38.3	60.1	42.9	0.0	6.3
	250	Cd2+ > Pb2+ > Cu2+ > Zn2+ > Ni2+	39.1	37.1	28.4	0.2	5.3
SS+AAC [1:1]	5	Cd2+ ≈ Pb2+ ≈ Cu2+ ≈ Ni2+ ≈ Zn2+	100	100	100	100	100
	10	Cd2+> ≈ Pb2+ ≈ Cu2+ ≈ Zn2+ > Ni2+	100	100	100	95.2	99.9
	60	Pb2+ ≈ Cu2+ > Cd2+ > Zn2+ > Ni2+	28.6	99.8	99.8	11.5	20.3
	100	Pb2+ > Cu2+ > Cd2+ > Zn2+ > Ni2+	26.8	59.9	39.3	0.2	5.4
	250	Pb2+ > Cd2+ > Cu2+ > Zn2+ > Ni2+	26.4	45.5	22.3	0.0	4.3
SS+AAC [1:4]	5	Ni2+ > Cu2+ > Zn2+ ≈ Pb2+ > Cd2+	99.4	99.5	99.9	100	99.8
	10	Cu2+ > Pb2+ > Zn2+ > Cd2+ > Ni2+	47.2	99.2	100	23.9	71.4
	60	Cu2+ > Pb2+ > Cd2+ > Zn2+ > Ni2+	21.9	97.6	98.5	0.0	6.5
	100	Pb2+ > Cu2+ > Cd2+ > Zn2+ > Ni2+	18.4	69.0	43.4	0.0	1.3
	250	Pb2+ > Cu2+ ≈ Cd2+ > Zn2+ > Ni2+	17.8	39.6	18.0	0.0	0.1
AAC	5	Pb2+ ≈ Cu2+ > Zn2+ > Cd2+ > Ni2+	41.3	100	100	44.7	71.1
	10	Cu2+ > Pb2+ > Ni2+ > Cd2+ > Zn2+	32.1	99.4	100	32.9	20.6
	60	Pb2+ > Cu2+ > Cd2+ ≈ Ni2+ > Zn2+	19.4	100	60.1	17.9	5.3
	100	Pb2+ > Cu2+ > Cd2+ > Zn2+ > Ni2+	15.0	60.4	41.0	2.9	4.2
	250	Pb2+ > Cu2+ > Cd2+ > Zn2+ > Ni2+	14.0	30.7	25.2	0.0	2.9

. For all tested adsorbents, the R values of metals became high in low L/S conditions and decreased with increasing L/S. Especially, the R values became low at L/S = 60, 100, and 250, except for the Pb²⁺ removal and the Cu²⁺ removal of SS and AAC grains at L/S = 60 (Figure 5). This suggests that control of L/S is an important factor to achieve high R values in multi-metal solutions, unlike the single-metal and binary-metal solutions.

As shown in Table 6, the selectivity sequences of metals in multi-metal solutions are highly dependent on the L/S for SS and AAC grains, and the Cd²⁺ and Pb²⁺ adsorptions are affected by the existence of competitive metal ions. The selectivity sequences for the mixtures of SS and AAC, on the other hand, were less dependent on the L/S compared to the results from single SS and AAC grains. For SS grains, the adsorption of Cd²⁺ (and Cu²⁺, Ni²⁺, and Zn²⁺) became much higher than that of Pb²⁺ at low L/S (=5 and 10), and the Pb²⁺ adsorption became higher than that of Cd²⁺. For AAC grains, the adsorption of Pb²⁺ (and Cu²⁺) became higher than that of Cd²⁺ at all L/S conditions in accordance with the tested results from single-metal and binary-metal solutions. For the mixtures of SS and AAC (SS+AAC [1:1]), the adsorption of Cd²⁺ became equal to the Pb²⁺ adsorption at low L/S (=5 and 10), but the sequence became the opposite at high L/S (=60, 100, and 250). Based on the test results above, therefore, the mixtures of SS and AAC grains are able to simultaneously remove Cd²⁺ and Pb²⁺ in multi-metals solution with high R values of approximately 100% at low L/S conditions (5 and 10). Again, the test results suggest the control of L/S is a key factor for the practical application of the mixtures of SS and AAC grains as adsorbents to remove Cd²⁺ and Pb²⁺ in wastewater.

In this study, SS exhibited a high capacity to adsorb Cd²⁺ from wastewater. For example, 0.105–2 mm particles of SS achieved effluent discharge standards of <0.001 mg/L for Cd²⁺ up to Ci < 1500 mg/L by the single-batch adsorption under the experimental conditions. Additionally, the SS+AAC (1:1) mixture removed 100% of Cd²⁺ and Pb²⁺ from binary and multi-metal solutions. However, the following facts should be considered in future studies and applications for the sustainable use of these low-cost adsorbents. Along with the metal adsorption process, SS and AAC grains released a relatively high concentration of Ca²⁺. Additionally, SS grains showed high alkalinity in water compared to AAC. Therefore, those factors should be controlled before treated water is discharged into the natural environment.

4. Conclusions

The results of batch adsorption experiments of the tested adsorbents revealed that SS grains had a high affinity for Cd²⁺ (>300 mg/g) but less affinity for Pb²⁺ (<20 mg/g). In the binary-metal solution, the mixtures of SS and AAC grains, especially SS+AAC [1:1] and SS+AAC [1:4], removed 100% of Cd²⁺ and Pb²⁺ simultaneously without any effect of metal molar mixed ratios at the L/S ratio of 60. Remarkably, in the multi-metal solutions, the metal R and selectivity sequence varied depending on the mixing proportions of SS and AAC grains and L/S values. It was found that the SS+AAC [4:1] and SS+AAC [1:1] mixtures of SS and AAC grains removed 100% of Cd²⁺, Pb²⁺, Cu²⁺, Ni²⁺, and Zn²⁺ simultaneously, at L/S = 10 and 60. The correlation regression showed that the ratios of released Ca²⁺ became greater than 1, meaning that approximately 2 and 6 times of Ca²⁺ were released in the one-metal adsorption process. For SS grains, active hydroxides (-OH) on the surface of metal oxides/hydroxides created a negative surface charge, resulting in the formation of an inner-sphere complex by replacing Ca²⁺. For AAC grains, the ion exchange between Ca²⁺ and Pb²⁺ on the adsorbent surface becomes dominant and the inner-sphere complex promotes the Pb²⁺ adsorption onto the surface with a negative charge. Thus, the amount of Ca²⁺ released became higher than the amount of metal adsorbed. Further studies are needed to examine the effects of other factors such as initial pH, temperature, and other competitive ions that control the heavy metal adsorption of SS and AAC grains. Additionally, the regeneration of metal-adsorbed adsorbents (i.e., collection of adsorption heavy metals from adsorbents) should be examined for practical application. The application of IBPs for the wastewater treatment, however, is essential with a high potential from the viewpoints of material efficiency and saving natural resource consumption.